This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-098871, filed Mar. 31, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a frequency change measuring device that measures changes in frequency to measure changes in a physical quantity.
Conventional methods for measuring a change in a physical quantity in terms of a frequency change involve using a variable-capacitance displacement sensor whose electrical capacitance is varied by a change in the physical quantity and measuring a change in an oscillator frequency caused by that sensor.
There are two such methods. The first method involves dividing the output frequency of the sensor which varies with a change of the physical quantity to define gating times and counting reference clock pulses within the interval between each gating time. The second method involves dividing a reference clock frequency within the measuring device to define gating times and counting the output frequency of the sensor within the interval between each gating time.
The first and second methods are common to each other in that a change in the sensor output frequency is measured in terms of a change in count value and an operation is performed on the count value to thereby determine a change in the physical quantity. Usually, the precise frequency measurements by those known methods require the reference clock frequency to be stabilized using a thermostatic oven or the like.
In FIG. 1 there is illustrated an example of a frequency change measuring device which implements such methods as described above. The measuring device is composed of a displacement detector 80 and an operations unit 70. The displacement detector comprises a variable-capacitance displacement sensor 81 which detects a change in a physical quantity as a change in its electrical capacitance and an oscillator 82 using the sensor as its frequency determining component. The operations unit 70 comprises a reference clock generator 71 which generates a clock signal as reference of measurement timing, a frequency divider 72 for dividing the output frequency of the reference clock generator, a gate circuit 73 for defining gating times from the output signal from the frequency divider, a counter 74 for counting the output frequency of the displacement sensor 80 in the interval between each gating time, a latch circuit 75 for holding the count in the counter, and an operations circuit 76 for performing operations on the count read from the latch circuit to determine the change in the physical quantity.
In the arrangement of FIG. 1 in which the oscillating frequency is varied in sympathy with a physical quantity, the resolution of frequency is improved by increasing the gating interval (window) between gating times, making it easy to detect variations in the physical quantity. However, as the gating interval becomes longer, it becomes more susceptible to variations and the magnitude of errors in frequency measurement increases. To decrease the magnitude of measurement errors, the gating interval is simply stabilized. To stabilize the gating interval, the reference clock generator is used put in a thermostatic oven. Power must be continuously applied to the operations unit 70 including the thermostatic oven during frequency measurement. The thermostatic oven needs large consumptive electrical power. In the field where a battery must be used as a power supply, therefore, measurements cannot be made over a long period of time.
In the above arrangement, if a cable used to send a signal from the displacement detector 80 to the operations unit 70 is long, then the signal will suffer attenuation within the cable. That is, since an oscillator output signal of the detector 80 is sent as it is to the operations unit 70 as the measuring signal, the signal is subject to attenuation. In particular, when the frequency of the measuring signal is high, the amount of attenuation is great and the distance over which the signal can be transmitted is short.
With the conventional frequency measuring device, as described above, when the gating interval is made long to increase the frequency resolution, it is susceptible to variation and hence the frequency measurement error increases in magnitude. The power must be applied continuously to the device which includes the thermostatic oven requiring large consumptive power during measurement. When the cable between the detector and the operations unit is long, the measured signal sent over the cable suffers considerable attenuation.
It is an object of the present invention to provide a frequency change measuring device which has no requirement for a thermostatic oven for stabilizing the frequency output of a reference clock generator.
According to a first aspect of the present invention, there is provided a frequency change measuring device including at least one frequency divider for frequency dividing a measuring signal whose frequency is varied by a change in a physical quantity to produce frequency-divided signals; at least one counter for counting the frequency-divided signals to calculate frequency-division numbers; at least one frequency division numbers transmitter for transmitting the frequency division numbers in synchronism with the frequency-divided signals; a frequency division numbers receiver for receiving the frequency division numbers receiver for receiving the frequency division numbers transmitted from the frequency division numbers transmitter to output the frequency division numbers; a reference clock generator for generating reference clocks; and an operations unit for determining a change in the frequency of the measuring signal on the basis of the frequency outputs of the reference clock generator and the frequency division numbers.
According to a second embodiment of the present invention, there is provided a frequency change measuring device including at least one frequency divider for frequency dividing a measuring signal whose frequency is varied by a change in a physical quantity to produce frequency-divided signals; at least one first counter for counting the frequency-divided signals to calculate frequency-division numbers; at least one frequency division numbers transmitter for transmitting the frequency division numbers in synchronism with the frequency-divided signals; a frequency division numbers receiver for receiving the frequency division numbers transmitted from the frequency division numbers transmitter to output the frequency division numbers; a second counter for counting an output of a reference clock generator which generates reference clocks; a latch unit for latching a count of the second counter on the basis of signals synchronous with the frequency division numbers from the frequency division numbers receiver; and an operations unit for determining a change in the frequency of the measuring signal on the basis of the count latched by the latch unit and the frequency division numbers.
The frequency change measuring device preferably further comprises a power supply for driving the frequency division numbers receiver, the second counter, the latch unit, and the operations unit, the power supply being turned on intermittently.
There may exist a plurality of measuring signals, the frequency divider, the first counter and the frequency division numbers transmitter each correspond in number to the measuring signals, and the frequency division numbers calculated from the measuring signals are received by at least one frequency division numbers receiver.
Preferably, the frequency divider, the first counter and the frequency division numbers transmitter are packed together as a frequency dividing unit, and the frequency division numbers receiver, the second counter, the latch unit and the operations unit are packed together as a frequency operations unit, and the transmission of the frequency division numbers from the frequency division numbers transmitter to the frequency division numbers receiver is made by radio communication.
Preferably, the reference clock generator operates in synchronism with a timekeeping device for the standard time.
Preferably, the device includes a scheduler for scheduling the on-off control of the power supply on the basis of the output of the operations unit.
In the frequency change measuring device of the present invention, to measure times at which the frequency-divided signals are produced, use is made of the frequency output of the reference clock generator synchronized with the timekeeping device for the standard time with little timekeeping error. Thus, even if the time interval between each frequency-divided signal is prolonged, no measurement error accumulates. For this reason, there is no need to stabilize the frequency output of the reference clock generator through the use of a thermostatic oven which requires large consumptive power, effecting a reduction in the power consumption of the measuring device and allowing a battery-powered frequency change measuring device to be manufactured which is compact and handy to carry about.
For example, when the frequency output of the reference clock generator is 1 MHz, the times at which the frequency-divided signals are produced can be determined with an accuracy of xc2x11.0 xcexcsec. When the frequency output of the reference clock generator is 1 KHz, the times at which the frequency-divided signals are produced can be determined with an accuracy of xc2x11.0 msec.
The standard time has been transmitted in the form of electromagnetic waves from communication satellites and ground stations and can be received all over the world. To receive the standard time, GPS clocks and radio-wave-based clocks are commercially available. These clocks are very precise and no timekeeping error accumulates with time. Thus, the frequency output of the reference clock generator is stable and hence is suitable for a time base for the reference time. For observation of physical phenomena in the fields where the electric power situation is bad, the frequency change measuring device of the present invention using a radio clock is handy and allows the standard time to be received with a small amount of power consumption. Thus, a compact, battery-powered frequency change measuring device can be implemented.
The frequency division unit sends the frequency division numbers resulting from frequency dividing a measuring signal to the frequency operations unit in synchronism with the generation of frequency-divided signals. The frequency division numbers contain information concerning changes in a physical quantity. The frequency division numbers are converted into bit signals, which in turn are sent to the frequency operations unit as low-frequency signals which can be transmitted even over a long cable. In the present invention, the frequency division numbers are transmitted by radio communication to the frequency operations unit in synchronism with the frequency-divided signals which also contain information concerning changes in the physical quantity.
The present invention can be adapted to a plurality of measuring signals. Thus, a single frequency operations unit can determine changes in a number of physical quantities or changes in a physical quantity at a number of points.
In the present invention, the power to the frequency operations unit is turned on intermittently to measure a change in the frequency of the measuring signal with low power consumption. The frequency operations unit precisely measures times at which the frequency-divided signals associated with the measuring signal were produced in the frequency division unit on the basis of the frequency output of the reference clock generator timed to the standard time clock device. Also, the frequency division numbers transmitted at those times can be known. Therefore, even when the frequency operations unit is intermittently powered, a change in the frequency of the measuring signal in the frequency division unit can be known from the times of production of intermittently obtained frequency-divided signals and the frequency division numbers at those times. A change in the physical quantity can be determined by performing operations on the frequency change.
In the present invention, since the frequency output of the reference clock generator synchronized with the standard time clock device is used, time errors are little accumulated. Thus, any change in the physical quantity can be detected by increasing the measuring interval. Even in the case where the frequency operations unit is operated intermittently, the difference between times at which the frequency-divided signals were produced can be known from the difference between frequency division numbers, allowing a change in the physical quantity to be determined with precision.
The present invention is especially useful in mounting a number of displacement sensors on the concrete walls of railroad or road tunnels or their surrounding bedrock and recording their movement over a long term at a point. The inventive measuring device, which can run on a battery over a long period of time, can detect long-term movement of the wall of tunnels and the surrounding bedrock and their abnormal movement which may result in collapse. In addition, the device does not require the battery to be exchanged often and is easy to maintain.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.